The present invention generally relates to mass flowmeters and in particular to a mass flowmeter having an arrangement for directly measuring a mass flow rate of a fluid.
In measurement of the flow rate of a fluid, it is desirable that the measured flow rate is represented in terms of mass so that the measured flow rate is not influenced by the type of the fluid, its physical property such as density, viscosity and the like, nor the condition of the fluid such as the temperature and pressure.
Conventionally, two types of mass flowmeters are used in measurement of the mass flow rate of the fluid. One of which is called indirect type which measures the volumetric flow rate of the fluid and converts the measured volumetric flow rate to the mass flow rate. Another type of the mass flowmeter is called a direct type which directly measures the mass flow rate of the fluid. Generally, the direct type mass flowmeter has a higher precision as compared with the indirect type mass flowmeter.
Various types of mass flowmeters are proposed on the basis of various principles for measurement of the flow rate in order to achieve higher precision. One of such flowmeter utilizes the Coriolis force which works on the fluid flowing through a vibrating tube for the direct measurement of the mass flow rate of the fluid.
For example, a mass flowmeter comprising a pair of U-shaped tubes connected to a flowmeter body having a fluid inlet and a fluid outlet is known. In this flowmeter, the pair of U-shaped tubes are vibrated or swung to a direction so as to alternatively increase and decrease the mutual distance therebetween. As a result of the vibration, the Coriolis force proportional to the mass flow rate of the fluid is applied to the fluid and the tubes are displaced. By detecting this displacement, the mass flow rate of the fluid is obtained.
In the aforementioned mass flowmeter utilizing the Coriolis force, it is difficult to manufacture the pair of tubes to a strictly identical size and shape. As a result, there tends to be a variation in the natural frequency of the respective U-shaped tubes. Therefore, it becomes necessary to compensate the variation of the vibratory characteristic of the tubes to coincide the natural frequency of the both of the tubes in order to improve the precision of the flow rate measurement. This compensation of the vibratory characteristic of the tubes is made by adjusting the balance weight on the tubes. However, the mounting and adjustment of the balance weight is a complex and tedious procedure.
In this type of mass flowmeter aforementioned, there arises another problem in that the U-shaped tubes extend transversally to the direction of a conduit on which is mounted the flowmeter. As a result, the U-shaped tubes occupy a large space which is substantially larger than the space for the conduit. Thus, such a mass flowmeter requires a large mounting space. Such a conventional mass flowmeter further requires a large flowmeter housing for accommodating the flowmeter unit and the U-shaped tubes. Therefore, the flowmeter cannot be positioned easily on a portion of a facility where other apparatus and devices are densely mounted. Instead, one has to painstakingly search for a suitable space surrounded by other apparatus and devices for accommodating the flowmeter housing.